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In general, a peep-hole; also Judas trap, Judas hole; listed as a sense of Judas in the OED. Used, oddly, in photography: the “Judas window” here is a set-up on some older, chiefly manual cameras which (by placement of a tiny mirror) allows the photographer to see in the viewfinder a setting which is made elsewhere on the camera, such as the aperture setting on the lens barrel.